The invention relates to a process for the separation by filtration of a crystalline silicate from the aqueous reaction mixture in which the silicate is present after the crystallization.
Crystalline silicates may be prepared starting from an aqueous mixture containing a silicon compound and alkali metal ions. The preparation takes place by maintaining the mixture at elevated temperature until the silicate has crystallized out and subsequently separating the crystals of the silicate from the mother liquor, washing, drying and calcining them. By incorporating compounds of certain trivalent metals into the aqueous mixture from which the silicates are prepared, crystalline silicates that contain the metals concerned may be obtained. Examples of such metals are aluminum, iron, gallium, rhodium, chromium and scandium. For the preparation of crystalline silicates with a certain crystal structure, organic cations such as quaternary alkylammonium ions are sometimes incorporated into the aqueous mixture from which the silicates are prepared. By varying the molar ratio in which the various reaction components are incorporated into the starting mixture the average crystallite size of the silicates to be prepared can be controlled.
Crystalline silicates are used on a large scale in industry, inter alia as adsorbents and as catalysts and catalyst carriers. In these applications the average crystallite size is often an important factor. As a rule, the crystalline silicates are more suitable for the above applications according as they have a smaller average crystallite size. In the preparation of fine-crystalline silicates on a technical scale, separation of the silicates from the aqueous reaction mixture in which they are present after the crystallization is a serious problem. It has been found that for the separation of fine-crystalline silicates from the aqueous mixture in which they are present after the crystallization, filtration is not a suitable process. When a filtering material with a relatively coarse structure is used, the crystalline silicate passes the filter unhindered. When a filtering material with a relatively fine structure is used, complete blockage of the filter by the crystalline silicate occurs very rapidly. It is true that the fine-crystalline silicates can be separated from the reaction mixture by centrifuging, but this separation technique is not suitable for use on a technical scale because of the high cost involved.
U.S. Pat. No. 3,516,786 discloses that in the preparation of crystalline aluminosilicates, improved crystallinity and a larger surface area are obtained by adding 0.1-20%v of a water-miscible organic solvent to the reaction mixture before crystallization. Netherlands patent application No. 7,610,763 (equivalent: G.B. Pat. No. 1,553,209) discloses that in the preparation of crystalline aluminosilicates, a high SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio can be obtained by including one or more alcohols in the aqueous base reaction mixture.
Applicants have now discovered that fine-crystalline silicates are easily separated by filtration from the aqueous mixture in which they are present after the crystallization, if it is ensured that the mixture which is subjected to the filtration contains an organic solvent that is miscible with water. The amount of organic solvent present in the mixture to be filtered should be at least 30%v, based on the mixture to be filtered. The organic solvent should be chosen from the group formed by monovalent alcohols, ketones, sulphoxides and cyclic ethers which, per molecule, contain at most five carbon atoms.